The present invention takes as its starting point a method for the heating and joining together, more particularly for the welding together, of workpieces by using a generator. During a heating interval, the generator supplies high-frequency energy to at least two electrodes disposed on the workpiece, the energy supply being controlled with respect to a target value by means of a predetermined time-related curve.
Such a method is known from German Patent DE-PS 858 870. There, high-frequency energy is supplied from a generator via an adjustable capacitive impedance to the electrodes on a workpiece and the high-frequency voltage at the electrodes is monitored. When it diverges from a target value, the capacitive impedance is altered with the aid of a motor and a regulation to the target value is thus effected.
During the welding operation the characteristics of the workpieces disposed between the electrodes will generally be altered. Certain zones change their properties or characteristics during the resulting temperature increase, those changes will alter the high-frequency energy absorption. It is also possible for workpieces to change position due to the contact pressure of the electrodes and, thereby, absorb a larger or smaller volume of high-frequency energy. Individual workpieces may exhibit variable welding behavior. It is thus possible that an electric breakdown may take place, or that the material may be inadequately heated and that a poor joining and connection of the workpieces may result. This may occur even when the high-frequency voltage is controlled in accordance with a predetermined time-related curve.
Furthermore, on account of varying material compositions in the joining zone, one frequently moves within the boundaries of a safe manufacturing progress. Due to varying degrees of humidity, uneven weight per unit area or inhomogeneous resin distribution, high-frequency breakdowns readily occur.
Such high-frequency breakdowns also occur in the case of a critical course of the joining zones in contiguous workpieces, e.g., in the door trim or instrument board of a motor vehicle. Reclaimed material possessing unintentional but noticeable differences in physical properties may sometimes be used for workpieces, e.g., for the carpet linings of the underbodies of motor vehicles. High frequency breakdowns are especially common when pieces that have differing physical properties are joined. For example, if a carpet covering and an anti-slip cover are welded together, the carpet covering is particularly susceptible to breakdown and may possibly show damage caused by fire.
Similar susceptibilities to welding treatment result consequent to other factors, such as the application of an adhesive, the type of adhesive if applied, the type of textile, or the treatment of the textiles in question, etc. These susceptibilities may also give rise to problems in the joining process.
Certain applications for vehicle assembly have been set forth above. Additional areas of application exist in vehicle assembly, such as the covering of electric components of the instrument board. These components are mainly fabricated from ABS (alkylbenzole sulfonate) and, in part, are joined together with the aid of a high-frequency joining process. These molded parts may possess varying wall thicknesses within the joining zone. In this context as well, it is of prime importance that work is carried out below the high-frequency breakdown limit.
Another area which may be considered for high-frequency heating is the manufacture of visors. In this context, an extraordinarily high degree of accuracy in the power control must be employed. Similarly, in the fabrication of theft-proof packaging (produced from molded, hard PVC) the joining zones have varying material thickness, which can readily result in production of flawed joints. Furthermore, this method, involving the use of high-frequency heating, is particularly advantageous for the drying, gluing, sterilizing, etc. of workpieces.
It is the object of the present invention to develop a method such that the ideal parameters for a secure joining together of the workpieces are adhered to; the adherence to ideal parameters effected with sufficient accuracy so that flawless products can be manufactured despite critical limitations values of the materials. Accordingly, the part rejection rate can be drastically reduced.